1. Field of the Invention
The present invention relates to a test device for detecting the presence of a constituent in a liquid test sample and method for making it. Moreover, it relates to minimizing the adverse effects of misuse of the device, thereby enhancing its accuracy and dependability.
The art of analytical chemistry has been greatly advanced since biochemistry began emerging as a primary scientific frontier, requiring increasingly sophisticated analytical methods and tools to solve problems, the solutions to which were never before attempted. Likewise, the medical profession has lent impetus to the growth of analytical chemistry, with its desiderata of both high precision and speed in obtaining results. This remarkable progress has been still further spurred by industries such as brewing, chemical manufacturing, and others.
To satisfy the needs of these expanding technologies, a myriad of analytical procedures, compositions and apparatuses have evolved, including solution chemistry techniques, automated machinery and the so-called "dip-and-read" type reagent strips. It is to the last of these that the present invention is primarily directed, although substantial benefit ultimately attaches to the other procedures as well.
Reagent strip test devices enjoy wide use in many analytical applications, especially in the chemical analysis of biological fluids, because of their relatively low cost, ease of utilizability and speed in obtaining results. In medicine, for example, numerous physiological functions can be monitored merely by dipping reagent strips into a sample of body fluid, such as urine, and observing a detectable response such as a change in color or a change in the amount of light reflected from or absorbed by the strip.
Compatible with such "dip-and-read" reagent strips have arisen many chemistries for detecting body fluid components. Many of these produce a detectable response which is quantitative or at least semi-quantitative. Thus, by measuring the response after a predetermined time, the analyst can obtain not only a positive indication of the presence of a particular constituent in a test sample, but also an estimate of how much of the constituent is present. Such strips provide the physician with a facile diagnostic tool as well as the ability to guage the extent of disease or bodily malfunction.
Illustrative of such strips currently in use are products available from the Ames Division of Miles Laboratories, Inc. under the trademarks CLINISTIX.RTM., MULTISTIX.RTM., KETOSTIX.RTM., N-MULTISTIX.RTM., DIASTIX.RTM., DEXTROSTIX.RTM., and others. Test devices such as these usually comprise one or more carrier matrices, such as absorbent paper, having incorporated with them a particular reagent or reactant system which manifests a color change in the presence of a specific test sample component. Depending on the reactant system incorporated with a particular matrix, these devices can detect the presence of glucose, ketone bodies, bilirubin, urobilinogen, occult blood, nitrite, and other substances. The specific color change and the intensity of the color observed within a specific time range after contacting the strip with the sample is indicative of the presence of a particular component and its concentration in the sample. Some of these test devices and their reactant systems are set forth in U.S. Pat. Nos. 3,123,443 (CLINISTIX.RTM.); 3,212,855 (KETOSTIX.RTM.); 3,814,668, 3,164,534 and 2,981,606 (DIASTIX.RTM.); and 3,298,789, 3,092,465, 3,164,534 and 2,981,606 (DEXTROSTIX.RTM.).
It is to those of the above-described devices having more than one reagent-bearing carrier matrix that the present invention is primarily applicable. Thus, a reagent strip can contain tests for more than one constituent in a particular liquid sample. For example, a single reagent strip could consist of a reagent-bearing carrier matrix responsive to glucose in urine, and another matrix spaced from but adjacent the first responsive to ketones, such as acetoacetate. Such a product is marketed by Ames Division under the name KETO-DIASTIX.RTM.. Another reagent strip marketed by Ames Division, N-MULTISTIX.RTM., contains eight adjacent reagent-incorporated matrices and provides analytical measurements of pH, protein, glucose, ketones, bilirubin, occult blood, nitrite and urobilinogen.
Despite the obvious, time-proved advantages of such multiple test reagent strips as these, misuse can result in misinformation. These multiple-analysis tools comprise complex chemical and catalytic systems, each reagent matrix containing a unique reactive system, responsive to its particular analysate. Thus it is possible, if the reagent strip is misused, for chemicals to be transported by the liquid sample being analyzed from one carrier matrix on the strip to another. Should this happen it is possible for reagents from one carrier matrix to interfere with those of the others so contacted, causing unreliable results. Although it is common in the reagent strip industry to provide detailed instructions as to how this problem is avoided, i.e., directions for properly manipulating the reagent strips, nevertheless ignorance or disregard of these instructions could permit reagents from one matrix to run over onto an adjacent one. It is to the prevention of this "runover" problem that the present invention is primarily directed.
The elimination of runover has been long sought after, but until the advent of the present invention, never adequately attained. Applicants' discovery, which was the culmination of an extensive research effort based on their initial conception of how to avoid runover interference, has finally solved this elusive problem, and the results are indeed as unique as the solution.
2. Discussion of the Prior Art
The patent literature is replete with accounts of myriad attempts at curtailing runover, the great bulk of the emphasis being directed to two basic concepts: the absorbence of runover liquid by bibulous layers placed beneath the reagent-bearing layers of reagent strips; and use of hydrophobic barriers between the spaced matrices to confine the sample liquid to the matrices. The former has met with moderate success, whereas the latter has not. But more importantly, neither has completely obviated the problem.
Of the multi-layer type of reagent strips, only one is described in the literature as successfully curtailing the problem of runover, and its teachings are hereby incorporated in the present disclosure by reference. Thus, U.S. application Ser. No. 872,560, filed Jan. 26, 1978, issued as U.S. Pat. No. 4,160,008 on July 3, 1979 assigned to Miles Laboratories, Inc., describes a test device in which the carrier matrices containing reagent formulations are provided with absorbent underlayers which are separated therefrom by sample-impervious barrier layers. Each matrix thus forms the upper layer of a laminate composite in which the barrier layer is disposed between the matrix and the absorbent base layer, the composite being fixed to a suitable support such as a plastic strip. When the test device is dipped into a liquid sample, the portion of the sample which would otherwise runover from one matrix to another is largely absorbed into the underlayer of the latter through the exposed sides, the barrier layer of the composite segregating the absorbed runover from the upper reagent layer.
No other art appears to be directed to the absorptive underlayer approach to solving the runover problem, although other multilayered reagent strip devices are described in which potentially incompatible reagents for the same test are separated from each other in layers and communicate upon wetting by the test sample. For example, U.S. Pat. No. 3,531,254 teaches that potentially incompatible reagents can be impregnated into separate layers to permit extended storage periods before use. When such a multi-layered matrix is wetted with a test sample, these layers communicate and the reagents previously separated become mixed to give the desired analytical test.
Another example of a multi-layered carrier matrix is the one shown in U.S. Pat. No. 3,802,842. Here, a porous pad containing no reagents abuts an upper pad containing the reagents for the desired test. Thus, when liquid sample is applied to such a carrier matrix some of the sample is absorbed by the non-impregnated pad, and some by the one bearing the reagents. As in the previous patent, the layers of this carrier matrix communicate with one another when wet. Some of the liquid (and some of the reagents) pass through the upper pad into the lower pad. There is no barrier provided between the two pads.
There exist other patents which, although less pertinent than the previous two, nevertheless are of interest when considering the present invention, and are mentioned here for the convenience and information of those interested in the present teachings. U.S. Pat. No. 3,418,083 depicts an indicator-impregnated absorbent carrier matrix treated with wax, oil or similar "hydrophobic" agents. It is said that when a sample of blood is placed on such a reagent strip, only the colorless liquid components permeate it, the proteinaceous, colored blood components remaining on the surface where they can be removed. Thus, it is taught, the liquid portion bearing the analysate permeates the reagent pad, whereas color interferants are precluded from it.
Still another prior art reference, U.S. Pat. No. 3,672,845 assigned to the present assignee, shows spraying adhesive onto a plastic or paper support member for the purpose of gluing on reagent-laden polymer particles. Yet another, U.S. Pat. No. 3,992,158, teaches an upper, semipermeable layer containing ascorbate oxidase affixed to a lower, reagent-laden layer.
Turning now to the second of the abovementioned attempts to curb runover--hydrophobic barriers between adjacent test areas of a reagent strip - there has been a not inconsiderable amount of patenting activity. U.S. Pat. No. 3,001,915, assigned to the present assignee, describes an absorbent paper reagent strip having spaced reagent-impregnated test areas for more than one sample component, each such area being separated from its neighbor by a non-absorptive barrier portion. The barrier is provided by impregnation of the paper strip with such materials as polystyrene, rosin, paraffin and various cellulose esters. The reagent strip is prepared, according to this reference, by impregnating a portion of a paper strip with a glucose-sensitive reagent system. When dry, a solution of one of the above barrier materials is applied to the paper adjacent the glucose-sensitive portion. After further drying a protein-sensitive reagent system is applied. The process is repeated, with alternate applications of reagent and barrier solutions with drying steps in between.
Yet an earlier patent, U.S. Pat. No. 2,129,754 issued Sept. 13, 1938, describes the impregnation of filter paper with paraffin wax whereby specific areas are left unimpregnated, but surrounded by the wax. These unwaxed spots can then be treated with indicator systems for a particular analyte.
U.S. Pat. No. 3,006,735 carries the concept of barrier material impregnated between reagent areas of a paper strip one step further by providing successive reagent areas responsive to different degrees of water hardness. Between these reagent areas are impregnated such water repellent materials as oils, waxes, silicones and printers varnish. Like the preceding two patents, this reference is restricted to paper or like bibulous material wherein reagent and barrier material alike are impregnated sequentially along its length.
Similarly, U.S. Pat. Nos. 3,011,874 and 3,127,281 teach the use of hydrophobic barrier materials impregnated in part of a paper strip in order to separate one reagent area from another to avoid contamination.
A product was recently marketed by Eiken Chemical Co. Ltd., of Tokyo, Japan which was a 4-reagent area dip-and-read test strip responsive to pH, protein, occult blood and glucose in urine. The strip comprised a long plastic support member which was a composite of a lower polystyrene layer and an upper polyvinylchloride (PVC) layer. The reagents were impregnated in paper pads which were affixed to the PVC side of the composite support member. Contact angle measurements with this product revealed a contact angle of about 108.degree. with distilled water. Since Applicants' first learning of this product, it appears that Eiken has withdrawn this configuration from the marketplace in deference to a new product whereby the PVC layer of the composite support member has been eliminated.
Finally, U.S. Pat. No. 3,964,871 mentions the separation of indicator reagent sites by non-absorbent or hydrophobic material.
Whereas the foregoing patents represent what is believed to be those most pertinent to the present invention, it should be noted that currently marketed reagent strip products for the most part comprise reagent-impregnated matrices affixed to a hydrophobic organo-plastic strip. Thus, the multiple reagent strip known as N-MULTISTIX.RTM., marketed by the Ames Division of Miles Laboratories, contains eight different reagent-impregnated matrices mounted on a strip of polystyrene film. Since polystyrene is hydrophobic, the reagent strip can be said to have hydrophobic interstices between adjacent matrices.
Despite the lip service given by prior art accounts of eliminating runover, the fact remains that there are presently no reagent strips commercially available capable of stifling this problem to anywhere near the extent achieved by the present invention. Of the patent art cited above, only that approach disclosed in U.S. Pat. No. 4,160,008, i.e., the use of an isolated absorbent underlayer, provides a real advance in the art. But even that approach, certainly widely divorced from the present invention, cannot approach the success at eliminating runover which the present invention achieves.
Prior art attempts using waxes, oils, silicones, etc. have not curtailed runover to a clinically significant extent; and what modest advances that may have been made were more than offset by serious drawbacks inherent to these attempts. For example, applying hydrophobic materials only at reagent area interstices embodies enormous technical problems, especially when compared with current techniques for manufacturing dip-and-read reagent strips. Besides the obvious extra steps required by intersticial application, there is the danger of some of the hydrophobic material overlapping the reagent areas--thus interfering with the paramount purpose of the device. Moreover, none of these prior art substances provides a suitable surface for adhesion. Small wonder no runover-free commercial products are available.
But even if these shortcomings were not prohibitive enough, the prior art hydrophobic substances lack the degree of hydrophobicity required to prevent runover. They do not provide a sufficient enough contact angle to achieve the required hydrophobicity, nor do they provide a suitable surface for binding either the absorbent matrices or the reagents themselves, were they to be coated directly on the hydrophobic surface. Only the present invention constitutes this long sought after breakthrough.
The present invention virtually eliminates cross-contamination between adjacent reagent areas of multiple test reagent strips. These results are truly incontrovertible. Nothing in the prior art approaches the dramatically high degree of success in solving this problem afforded by the presently disclosed concepts.
But the contribution of this discovery to the state of the art goes beyond the elimination of runover. Surprisingly, it has been found that adhesive techniques currently used for attaching reagent matrices to a polystyrene base support provide even stronger adhesive bonds when the present invention is utilized. Moreover, it is not necessary to utilize expensive process steps such as depositing hydrophobic coats between adjacent matrices. These and other advances in the current state of the art will become evident in view of the present specification and claims.